With winter approaching, I know I will have to use more photographs as reference and tools for my work. It doesn't particularly thrill me as I frankly prefer to work from reality. Making sketches on location allows me to observe many details, such a the correct light and colors, shadows and their color too, the overall landscape rather than a framed one, I can move around to see what's behind a rock, even far, I can jot down notes about the true green of trees, and much more. Using watercolor helps a lot too.
So, photographs limit the artist vision. First, they never truly represent the colors, and the lighting can vary. A camera will never replace your eyes. Photographs are also flat, where the world is 3-dimensional after all. Finally, they are tiny ! When looking at a landscape, the scale isn't comparable and it can be difficult to work from an 8x10 print or smaller if you don't know how to scale it down accurately when you do your preliminary sketches.
That said, they are valuable for other reasons. When cold temperatures hit and snow covers the ground, sketching or painting on the spot isn't easy. That's where photographs come in the picture, no pun intended ! They are a good tool, but you need to keep an editing and evaluating eye while using them.
Finally, it's important to use only your own photographs, first from a copyright point of view, but also because when you take them, you will be better able to translate the emotion from it onto canvas or paper than using someone else's. You did take that photo, your mind and eye remember it, and translating it will be much easier than using one that you know nothing about.
But if given a choice, do work on site if you can, it will make you a better artist.